Monitoring of chemical composition in harsh environments including downhole and underwater conditions is critically important for a range of fossil energy related applications, which include unconventional, deep and ultra-deep oil and gas resource recovery through drilling and hydraulic fracturing techniques, as well as environmental monitoring in reservoirs for CO2 sequestration. These conditions represent extremely challenging environments for the development and deployment of sensing technologies due to an aggressive combination of temperature and pressure, as well as the presence of chemically corrosive chemical species and a potentially high salinity. Temperatures ranging up to 300° C. and pressures ranging up to 30,000 psi can be relevant for these applications depending on the specific environment. Such temperatures and pressures are beyond the limit of most electrical and electronic components used in sensor applications, due in many cases to the instabilities associated with packaging, wires, and interconnects. For this reason, approaches that eliminate the need for electrical components and connections at the sensing location can also eliminate a common mode of failure for conventional sensor devices.
Optical based sensing methodologies offer this advantage and can also be advantageous from a safety perspective in the presence of potentially flammable gas and chemical species. In particular, sensors that employ fiber-bragg gratings inscribed into specialty optical fibers capable of withstanding the temperature and pressure conditions of interest have already been deployed commercially for distributed pressure and temperature sensing. In contrast, optical fiber based sensors for subsurface chemical sensing applications have not been commercially deployed due in part to the lack of optical sensor elements with useful, reversible, and rapid responses to particular chemical species of interest.
While a broad range of parameters related to the chemistry of harsh conditions such as downhole conditions can be potentially monitored, pH is a key parameter whose accurate measurement at downhole wellbore conditions is critical in understanding formation fluid water chemistry to predict corrosion and scale potential. Because gases and solids can come out of solution as downhole samples are transported to surface laboratories, it is important to develop technologies for accurate pH measurements downhole in the native condition at reservoir temperatures and pressures. The lack of a robust measurement requires large safety margins in the selection of corrosion resistant materials and a significant economic impact can therefore be realized by the development of such a technology. In addition, measured pH values can be utilized to infer additional information about the chemical composition of a fluid such as the concentration of CO2 in fluids contained within geological formations for CO2 sequestration.
A broad range of technologies exist for pH sensing in aqueous conditions including pH sensitive dyes, electrochemical and potentiometric based approaches, and electronically conductive polyaniline-based polymers. Additionally, plasmonic sensors have been demonstrated in which noble metals are functionalized with capping agents or an organic matrix that mediates a response to pH through relatively large changes in swelling of the polymer, modification of refractive index or through protonation/deprotonation reactions, or aggregation and de-aggregation of particles in solution. See e.g. Mishra et al., “Surface plasmon resonance based fiber optic pH sensor utilizing Ag/ITO/Al/hydrogel layers,” Analyst 9 (2013); see also Singh et al., “Fabrication and characterization of a highly sensitive surface plasmon resonance based fiber optic pH sensor utilizing high index layer and smart hydrogel,” Sensors and Actuators B 173 (2012); see also Asian et al, “Enhanced Ratiometric pH Sensing Using SNAFL-2 on Silver Island Films: Metal-enhanced Fluorescence Sensing,” Journal of Fluorescence 15(1) (2005); see also Toh et al, “Induced pH-dependent shift by local surface plasmon resonance in functionalized gold nanorods,” Nanoscale Research Letters 8 (2013). Optical sensors based on protonation of silica-based sol-gel materials have also been reported. See Rayss t al., “Ion adsorption in the porous sol-gel silica layer in the fibre optic pH sensor,” Sensors and Actuators B 87 (2002); and see Rayss et al., “Optical Aspects of Na+ Ions Adsorption on Sol-Gel Porous Films Used in Optical Fiber Sensors,” Journal of Colloid and Interface Science 250 (2002). However, these silica gel materials required coating on a highly bent optical fiber to be effective which is undesirable for pH sensing applications due to limitations in sensor design including distributed interrogation. Similarly, the silica gels were utilized for pH sensing without a high temperature pretreatment significantly above the subsequent temperature at which sensing experiments are performed, which thereby would limit the stability of the silica gel sensing material to near-ambient temperature applications to avoid modifications to the silica based layer during the sensing experiment. Further, pH detectors which have previously incorporated optically active nanomaterials rely a supporting matrix where the matrix itself exhibits a change in surface charge density over a given pH range.
It would be advantageous if a measurement methodology allowed for mapping of information about pH in real-time spatially within harsh conditions such as wellbores and throughout geological formations. It would also be advantageous if the sensing approach was optical-based in nature with a sensing response that was not dependent upon protonation and deprotonation of an organic indicator dye, due to inherent limitations in both temperature stability and resistance to leaching. Higher stability sensing materials are desired for long-term operation in aggressive downhole environments. As such, it would be preferred to use alternative sensing materials that exhibit chemical and temperature stability but demonstrate a reversible response to changing pH conditions.
Provided herein is a method of pH sensing which addresses these weaknesses by exploiting the optical property changes of inorganic oxide based nanoparticles that are stable under harsh conditions. The method exhibits a strong overall optical response associated with reversible interactions between the pH sensing material and the solution for which pH is being monitored. Exploitation of the inorganic oxide based nanoparticles as the absorption-based indicator elements to replace organic dyes potentially allows for a broader application space, improved temperature stability, and the possibility of multi-parameter monitoring through broadband wavelength interrogation by monitoring changes in optical properties in response to other important parameters such as temperature. The application of nanoparticle based oxides also allows for modifying the corresponding wavelength dependence and magnitude of the optical response through tailoring particle size and shape. In some embodiments, optically active elements can be incorporated within the inorganic oxide nanoparticles having characteristic optical properties such as metal nanoparticles to form so-called core-shell nanoparticle structures.
These and other objects, aspects, and advantages of the present disclosure will become better understood with reference to the accompanying description and claims.